1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and an image forming program product. More particularly, the present invention relates to an image forming apparatus, an image forming method, and an image forming program product that create an image by a plurality of exposure light sources.
2. Description of the Related Art
In recent years, in an electrophotographic image forming apparatus such as an MFP (Multi Function Peripheral), in order to improve resolution, a multi-beam exposure method is often adopted in which by using a plurality of exposure light sources an image is created on a single image carrier. A representative multi-beam exposure method includes a two-beam exposure method in which laser beams are emitted from two exposure light sources.
When a multi-beam exposure method is adopted, variation in density resulting from differing sizes of dots created by laser beams may occur in an output image.
When printing is performed in an image forming apparatus that uses a multi-beam exposure method, because of differing intensities of lasers from light sources, variation in density may occur in an output image. To prevent the variation in density, the intensity of a laser from each light source is adjusted upon production. Japanese Laid-Open Patent Publication No. 2004-276582 (hereinafter, referred to as Patent Document 1) discloses an adjustment method in which by comparing image patterns, each composed of light from each light source, uniform density is obtained. Specifically, the adjustment method disclosed in Patent Document 1 is a method of correcting changes in the amounts of exposure from a plurality of light sources due to deterioration, by using an actual image by a laser beam from each light source. Upon the adjustment, each light source is fully exposed to create an image (solid image) and each image is outputted. In the method, to create the solid image, the number of rotations of a polygon motor is changed in such a manner that the interval between exposure-scanning by a laser beam from one light source to a photoconductor and exposure-scanning by another laser beam from a subsequent light source to the photoconductor is shortened.
The applicants of the present application have found that when in an image forming apparatus that uses a two-beam exposure method, a patch with a tone close to highlights is reproduced by using an error diffusion method, a bias of dots (referred to as isolated dots) in which a single light emission waveform by only one laser is included in a single dot occurs in a certain area and in an output image the bias of the isolated dots is conspicuous as woodgrain variation in density.
Here, given that a single light emission waveform of a beam from one light source LD1 is one shown in FIG. 12A and a single light emission waveform of a beam from the other light source LD2 is one shown in FIG. 12B, even if light outputs at peaks are the same, due to differing responsivities of the light sources, a difference occurs in the leading edge of a waveform before reaching a peak.
An area of a single light emission waveform obtained by a light emission output P and a time T represents a density. When differing light emission waveforms, such as those shown in FIGS. 12A and 12B, are included in a single dot, a difference in density occurs between an isolated dot by light source LD1 and an isolated dot by light source LD2. Hence, if any of isolated dots is biased, variation in density may occur in an output image.
Although it is verified that the variation in density is conspicuous when an error diffusion method with which a bias of dots is likely to occur is used, even when a screen tone reproduction method is used, if such a parameter that dots created by only one of lasers are biased in a certain area is selected, it is obvious that similar variation in density occurs. Hence, to prevent such variation in density that has been found by the applicants of the present application, an adjustment needs to be made such that identical reproduction of dots by lasers from light sources is obtained.
However, the method of adjusting the intensities of lasers from light sources, such as the one disclosed in Patent Document 1, is a method of adjusting the intensities of lasers when the light sources are fully exposed, and is not a method of making an adjustment such that identical reproduction of dots by lasers from light sources is obtained. That is, even if the interval between one exposure-scanning and subsequent exposure-scanning is shortened as is disclosed in Patent Document 1, an adjustment cannot be made such that identical reproduction of dots by lasers from light sources is obtained, causing a problem that variation in density resulting from that occurs.